In a continuous ink jet printer, ink is ejected continuously through a nozzle as a single jet of ink which is then broken up into a stream of substantially uniformly sized and spaced apart droplets, typically by applying pressure pulses to the ink or by vibrating the nozzle. The droplets are then caused to travel either into a collection gutter (in which case they are not printed) or are allowed to fall onto the surface on which the ink is to be applied. Typically, this is done by charging the droplets and then passing them through a deflecting electric field. The strength of the deflecting field and/or the charge on the droplets is varied to cause the line of flight of the droplets to be printed to depart to differing extents from the flight line to the gutter so that they miss the gutter and are printed at different positions on the substrate depending upon the extent to which they are deflected. Alternatively, the gutter is off set from the straight line of flight and the field deflects the droplets into the gutter at one extreme of the deflection. The term "ink jet printer" is used herein to denote the above type of printing apparatus.
The droplets which are collected in the gutter are not printed and are re-cycled to the ink reservoir serving the print head for re-use. However, during their flight and re-cycle, some of the solvent or carrier medium for the ink is lost from the droplets through evaporation.
The proper operation of the print head is dependent, inter alia, upon the viscosity of the ink flowing through the nozzle orifice and this is altered by the loss of solvent or carrier medium from the ink. The viscosity is also affected by the temperature at which the print head is operated and the composition of the ink, both of which latter can vary from print run to print run. It is therefore necessary to ensure that the viscosity of the ink is maintained within desired limits at all times and these limits may not be the same for each print run.
In practice it has proved difficult to maintain a uniform and consistent viscosity, since the losses of solvent or carrier medium are not consistent and vary considerably with the ambient conditions as well as with the relative proportion of droplets which are recycled to those which are printed.
It has been proposed to measure the weight lost from the ink in the reservoir of the ink jet printer; to allow for the proportion of that loss which is due to the ink in the droplets which have been printed; and then to add solvent or carrier medium to make up the apparent difference, assuming that this difference is made up totally by lost solvent or carrier medium. However, this is cumbersome and requires that the print head be shut down so that all ink in the system is returned to the reservoir for weighing. Furthermore, it does not take into account any extraneous losses or changes in temperature which can affect the viscosity of the ink.
In another method proposed in Japanese patent application No. 21723/1979, the rate of flow of ink is measured using a flow meter in a bypass line in the print head which is fed with ink under substantially constant pressure. Since the flow rate will vary with the viscosity, this gives an indication of the viscosity variations. However, if the pressure at which the ink is fed varies, this can mask any effect a viscosity change may have and accurate measurement and control of the feed pressure is required. Furthermore, flow meters require moving parts and do not give accurate readings at the comparatively low volume flow rates which are normal in ink jet printers.
It has also been proposed in European patent application No. 228828 to measure the time taken for ink to flow through a restricted inlet into a vessel and to fill that vessel between known lower and upper limits. However, such a system can only be operated if the pressure at which the ink is fed top the vessel is substantially constant. The system can anly be operated intermittently and requires extra controls for the operation of the filling and emptying of the vessel and the timing of the filling cycle.
It has also been proposed in European Application No. 0123523 to measure the pressure drop along a duct in the ink circuit when a valve in that duct is opened, the pressure drop being proportional to the viscosity of the ink in the duct. Again, that method cannot be operated continuously and requires control systems for the valve and requires a constant and/or accurately known volume flow of fluid to the duct, which is difficult to achieve in practice.
U.S. Pat. No. 3,938,369 describes a system in which the pressure drop along a capillary tube is measured at a constant volume flow rate through the tube. The pressure drop will vary with a change in the viscosity of the fluid, but this can only be monitored if the flow of fluid is held constant as required in this system. If the flow is allowed to vary, then the pressure drop due to that variation may mask any variation due to viscosity.
We have now devised a simple method for monitoring the change in viscosity of fluid flowing through a duct using a simple device which requires no moving parts. The method of the invention is not dependent upon maintaining a constant and/or accurately known pressure or volume flow rate through the duct, as has been required with the prior proposals. The viscosity can be monitored over a wide range of conditions with a simple device on a continuous basis and can be used to control the addition of solvent or carrier medium to the ink in an ink jet printer to optimise operation of the printer.